The present invention relates to a gas turbine combustor in which air and fuel are preliminarily mixed with each other. Particularly, the gas turbine combustor is capable of lowering the concentration of NOx contained in a gas exhausted from a gas turbine.
A gas turbine plant and a combined cycle power plant each includes a plurality of gas turbine combustors. A combustion gas burnt in the gas turbine combustors is guided to gas turbines so as to drive the gas turbines. It has been known that the heat efficiency of the turbine of a gas turbine plant of the described type is improved when the temperature of the inlet portion of the turbine is raised. In order to improve the heat efficiency of the turbine, it is necessary to raise the temperature of the inlet portion of the turbine.
In the gas turbine combustor, the temperature of the combustion gas is variously limited due to the heat resisting limit of the gas turbine and the material of the combustor. Furthermore, the necessity of taking a countermeasure against NOx (nitrogen oxides) in the gas turbine combustor causes the temperature of the combustion gas to be limited.
A main cause of generation of NOx in the gas turbine combustor is a local rise of the temperature of the combustion gas in the gas turbine combustor. The quantity of generated NOx depends on the temperature of the combustion gas in a combustion region in the gas turbine combustor. NOx is generated in a large quantity in a case where fuel and air are brought to a diffusion combustion at a high temperature near an adiabatic flame temperature in a state of an equivalent ratio of about 1 of fuel and air.
As a method of preventing the generation of NOx in the gas turbine combustor, a lean pre-mixture combustion method is available in which fuel and air are previously mixed with each other and burnt in a lean fuel state.
A gas turbine combustor employing the lean pre-mixture combustion method has been disclosed, for example, in Japanese Utility Model Publication No. HEI 4-43762. The gas turbine combustor, as shown in FIG. 6, has an arrangement in which a pilot fuel is, in addition to the previous mixture of the main fuel, mixed to reduce the diffusion combustion generating NOx in a large quantity to reduce NOx significantly.
The conventional gas turbine combustor shown in FIG. 6 has a combustor liner 1 divided into a first-stage combustion region 2 and a second-stage combustion region 3. A pilot fuel nozzle 4 is disposed in the head portion of the combustor liner 1, the fuel nozzle 4 supplies pilot fuel A to the first-stage combustion region 2.
A pre-mixing duct 6 for previously mixing main fuel C injected with air through the main fuel nozzle 5 is disposed around the combustor liner 1. The main fuel C mixed previously in the pre-mixing duct 6 is injected into the second-stage combustion region 3 to be burnt.
On the other hand, the pilot fuel nozzle 4 has, in the central portion thereof, a fuel passage portion 4a extending in the axial direction of the pilot fuel nozzle 4. An air passage portion 4b is disposed to surround the fuel passage portion 4a. At the inlet port and an outlet port (the inlet port for the liner) of the air passage portion 4b, swirlers 7 and 8 for swirling air flows are disposed. According to the structure in an arrangement in which the pilot fuel A is injected into each of the swirlers 7 and 8 or the swirler downstream portion.
As shown in FIG. 7, the operation of the conventional gas turbine combustor is performed in a manner such that a combustion operation using only pilot fuel A injected through the pilot fuel nozzle 4 is performed from ignition to a state in which the gas turbine load becomes somewhat partial load. At this time, the flow rate of the pilot fuel is controlled by one fuel control valve 9 and the pilot fuel is supplied to the pilot fuel nozzle 4. The pilot fuel nozzle 4 divides the pilot fuel into pilot diffusion fuel a and pilot pre-mixed fuel b.
The pilot diffusion fuel a is diffused by the swirler 8 and supplied to the first-stage combustion region 2 to be burnt. On the other hand, the pilot pre-mixed fuel b is, in the air passage portion 4b, uniformly mixed with air to be injected into the first-stage combustion region 2 through the swirler 8 to be subjected to combustion.
At this time, the fuel distribution of the pilot diffusion fuel a and the pilot pre-mixed fuel b is determined by the area of each of the fuel injection ports. In order to reduce NOx, the passage area of each of the swirlers 7 and 8 and the air passage portion 4b is made to be relatively large in order to sufficiently lower the fuel-air ratio (weight flow rate of fuel/weight flow rate of air).
When the operation of the gas turbine in a heavy load region has started, the fuel control valve 9 is throttled to decrease the pilot fuel A so as to lower the diffusion combustion ratio as shown in FIG. 7. Furthermore, a fuel control valve 10 is opened to supply the main fuel C to the main fuel nozzle 5. The supplied main fuel C is uniformly mixed in the pre-mixing duct 6, and then, it is injected into the combustor liner 1 to be burnt in the second-stage combustion region 3. In the pre-mixing duct 6, a passage area is maintained through which air capable of sufficiently previously lean-mixing the main fuel, which occupies 70 to 80% of the overall fuel rate, flows.
Since a portion of the pilot fuel A is, in the conventional gas turbine combustor, previously lean-mixed in addition to the main fuel, the ratio of the pilot diffusion fuel a can be lowered. As a result, NOx can significantly be reduced. However, the ratio of the diffusion fuel a is determined by the flow rate of the pilot fuel A and, therefore, the lowering thereof is limited to about 20% of the overall flow rate. The ratio cannot be further lowered and accordingly a limitation is present to further reduce NOx.
Another example of a conventional gas turbine combustor has been disclosed, for example, in Japanese Patent Laid-Open Publication No. HEI 4-98014. An example of the gas turbine combustor of this example is shown in FIGS. 8 and 9. The gas turbine combustor has a basic structure substantially the same as that the gas turbine combustor shown in FIG. 6, in which a combustion chamber formed in the combustor liner 1 is divided into a first-stage combustion region 2 and a second-stage combustion region 3 formed downstream side thereof. A plurality of pre-mixing ducts or pre-mixing pipes 6 are disposed around the combustor liner 1, the pre-mixing ducts 6 previously uniformly mixing main fuel and air in a lean fuel state, followed by injecting the main fuel previously mixed into the second-stage combustion region 3 to burn the main fuel.
The gas turbine combustor shown in FIG. 8 has a pilot fuel nozzle 4 composed of only a diffusion fuel nozzle so that pilot fuel A injected through the pilot fuel nozzle 4 is formed into a swirling flow by a pilot combustion swirler 8. The swirling flow is guided by an annular swirling flow guide 11 disposed downstream the pilot fuel nozzle 4 so that a stable combustion in the first-stage combustion region 2 formed at the central position of the swirling flow guide 11 is realized.
Since also the stable combustion can be also achieved in a state where the quantity of the pilot fuel A is relatively small at the pilot diffusion combustion, the diffusion combustion performed by the pilot fuel nozzle 4 is decreased and the pre-mixing combustion can be performed by the main fuel nozzle 5 with substantially preventing generation of NOx so as to realize significant reduction of NOx.
On the other hand, the gas turbine combustor shown in FIG. 9 comprises the pilot fuel nozzle 4 of the gas turbine combustor shown in FIG. 8 which is partially formed into a pre-mixing structure to decrease the NOx.
The described gas turbine combustor has a pilot pre-mixed fuel nozzle 7, additionally disposed upstream from the pilot combustion swirler 8, that gives a swirling flow to the pilot fuel A injected through the pilot fuel nozzle 4. Since the pilot pre-mixed fuel nozzle 7 injects the pilot pre-mixed fuel b followed by mixing it in the air passage portion 12 in a lean fuel state and burning the pre-mixture in the first-stage combustion region 2, the generation of NOx can be reduced. At this time, the pilot diffusion fuel a is decreased as compared with that in the gas turbine combustor shown in FIG. 8, thus realizing the reduction of NOx.
In recent gas turbine plants, the temperature of the combustion gas in the gas turbine combustor is raised to further improve the heat efficiency of the gas turbine. With the trend of raising the high temperature of the combustion gas, it is further required to reduce NOx. In order to realize an aimed value of reducing NOx, development of a low-NOx gas turbine combustor is desired in which the diffusion combustion, which generates NOx in a large quantity, is restricted to several % of the overall combustion quantity and the residual portion is fully burnt in a pre-mixed and lean state with generation of NOx substantially prevented.
The conventional gas turbine combustor shown in FIG. 6 has the structure designed in order to sufficiently cause the pilot pre-mixed fuel b to be a lean fuel mixed state in which air in a relatively large quantity is allowed to flow through the pilot pre-mixed fuel nozzle 7 and the air passage portion 4b. Therefore, the pilot combustion swirler 8 is enlarged relatively in size, and therefore the total size cannot easily be reduced. Accordingly, the reduction of the pilot diffusion fuel a to about several % of the overall flow rate of the fuel provides a problem of instable combustion, such as incomplete combustion or misfire. It therefore becomes impossible to perform an operation in which the pilot diffusion combustion is changed from about 30% to about several % of the overall fuel flow rate with a pressure difference through the fuel injection port maintained by one pilot fuel nozzle 4.
Furthermore, the gas turbine combustor shown in FIG. 9 has a problem caused from a similar problem that the pilot diffusion fuel a cannot be reduced to about several % of the overall quantity of the fuel.
The gas turbine combustor shown in FIG. 8 has the structure in which the pilot fuel nozzle 4 is composed of only the diffusion fuel nozzle. Therefore, an operation with only the pilot diffusion combustion which generates NOx considerably is performed until the gas turbine load reaches a load with which the pre-mixture combustion with the fuel C can be commenced.
The operation with only the pilot diffusion combustion is designed to maintain the diffusion combustion by introducing, in a relatively large quantity, air required for the diffusion combustion through the pilot combustion swirler 8. Accordingly, in the gas turbine combustor shown in FIG. 8, the ratio of the pilot diffusion fuel cannot be lowered in a heavy gas turbine load region in which the main fuel C is introduced and the pre-mixed combustion that generates NOx in a small quantity is commenced. If such ratio is lowered to about several %, the instable combustion, such as the incomplete combustion or the misfire, cannot be prevented.
Furthermore, the conventional gas turbine combustor is not provided with a specific air passage portion and a flame holding mechanism for performing the pilot diffusion combustion which is several % of the overall fuel flow rate, thus being defective and inconvenient.